The superior mechanical properties of polyoxymethylene (POM) molding compositions are the reason for their use in numerous applications. To improve their properties, the polyoxymethylene homo- and -copolymers are provided with additives to adapt the properties to the application, for example by using reinforcing fibers.
The effect of these additives on the properties of the molding composition is affected by the coupling of the additive to the plastics matrix. Attempts to couple glass fibers to a polyoxymethylene matrix are known in the prior art.
DE 2162345 discloses a thermoplastic composition comprising a polyoxymethylene, an isocyanate coupling agent and reinforcing glass fibers wherein the glass fibers are sized with aminoalkylsilane compounds. The diisocyanate coupling agent is used to improve the compatibility of the polyoxymethylene matrix with the reinforcing fibers.
Isocyanate coupling agents are highly reactive with nucleophilic groups such as OH or NH2 groups. Therefore, the use of further additives to reinforce polyoxymethylene compositions which comprise coupling agents on basis of isocyanates are limited.
US 2005/0107513 tries to avoid these problems and uses a catalyst which catalyses the chemical reaction between the polyacetal matrix polymer and the surface of the additive, i.e. the glass fiber. Thus, the use of a coupling agent is avoided. However, coupling agents such as isocyanates are very effective and contribute to the mechanical properties of the fiber reinforced polyoxymethylene compositions. On the other hand, sensitive additives which can react with the coupling agents were believed to be avoided. Consequently, additives which reduce the formaldehyde emission have not been used in the prior art for fiber reinforced polyoxymethylene molding compositions due to the presence of highly reactive isocyanate coupling agents.
POM has also been used to produce long glass fiber POM composites. Standard POM has shown the same poor adhesion to long glass fiber as is seen with short glass fiber. A way to overcome the adhesion problems is to use an ethyltriphenylphosphoniumbromide catalyst to promote the adhesion of standard POM to the long glass fiber as described in patents EP-B1-1483333 and U.S. Pat. No. 7,169,887 B2. The catalyst technology improves the mechanical strength of the POM/long glass fiber composites, compared to standard POM/long glass fiber composites, but the product still does not have sufficient tensile strength for some applications such as seat rails for automobiles which require good tensile strength and good flex fatigue and creep performance.
The object of the present invention is the provision of a fiber reinforced polyoxymethylene composition which demonstrates improved mechanical properties while having low formaldehyde emissions.
A further object of the invention is the provision of fiber reinforced polyoxymethylene compositions which show an excellent tensile strength while having a good flex fatigue and creep performance.